Sometimes plugins require a specific feature implemented in the Redmine core or the plugin overrides a specific view which requires you to control on which (specific) versions of Redmine the plugin can be installed to assure that the required core is available. Such prevents a lot of issues regarding plugin-compatibility.

The above can be accomplished by utilizing the @requires_redmine@-method (see issue #2162 for the implementation dicussion and it's actual implementation in r2042). Utilisation of the method provides an easy, reliable way to create plugins that require a specific version of Redmine and which are setup to stop Redmine with a message about a non-supported version if the version-requirement is not met.

Due to the fact that it is so easy to extend models and controllers the Ruby way (via including modules), Redmine shouldn't (and doesn't) maintain an API for overriding the core's models and/or controllers. Views on the other hand are tricky (because of Rails magic) so an API for overriding them is way more useful (and thus implemented in Redmine).

To override an existing Redmine Core view just create a view file named exactly after the one in _../app/views/_ and Redmine will use it. For example to override the project index page add a file to _../vendor/plugins/my_plugin/app/views/projects/index.rhtml_.

A quick example of *adding a new method* can be found on Eric Davis' "Budget plugin":http://github.com/edavis10/redmine-budget-plugin/blob/5076b1c88b57c2068aa92cdf694769dbd22d061a/lib/issue_patch.rb. Here he added a new method to Issue called @deliverable_subject@ and also declared a relationship.

A quick example of *wrapping an existing method* can be found on Eric Davis' "Rate plugin":http://github.com/edavis10/redmine_rate/blob/4666ddb10e1061ca3ef362735d0d264676b99024/lib/rate_users_helper_patch.rb. Here he uses the @alias_method_chain@ to hook into the UsersHelper and wrap the @user_settings_tabs@ method. So when the Redmine Core calls @user_settings_tabs@ the codepath looks like: